Chromium electroplating



. tions.

mium solutions.

United States Patent Delaware No Drawing. Filed Apr. 25, 1966, Ser. No. 522,361 Claims. (Cl. 204-51) This application is a continuation-in-part of application Ser. No. 308,069 filed Sept. 11, 1963, now abandoned.

This invention relates to the electrodeposition of chromium fror aqueous acidic hexavalent chromium solu- More particularly it relates to the use of certain fluorocarbon phosphonic acids in such plating solutions, and also to the use of mixtures of these phosphonic acids and certain fluorocarbon sulfonic acids as activating agents in conjunction with low concentrations of catalyst ions especially low concentration of sulfate ion to make possible operationally simplified chromium plating baths of improved covering power in recessed areas (low current density areas). Not only is the covering power improved, but the tendency to form strong discoloration or iridescent films where the chromium plate leaves off in the very low current density areas is appreciably decreased.

It was early established that small amounts of certain catalyst anions such as sulfate are required in order to electrodeposite chromium from acidic hexavalent chro- Inthe progress of the art of chromium plating, the use of mixed'catalyst'anions, such as combinations of sulfate ions with either fluoride, fluosilicate, fluoborate, flualuminate, fluotitanate, fluozirconate or boric acid were employed to make possible chromium plating baths of improved covering power and of simpler control. Patents 1,844,751, 2,063,197, 2,042,611, 2,640,022 and 2,952,590. In particular, reference is made to Luken U.S. 2,042,611; Passel US. 2,640,021 and Stareck US. 2,640,022 and 2,952,590, for the development of the selfregulation feature of the catalyst acid anion radical content of chromium plating baths. Lukens used saturation concentrations of the slightly soluble strontium sulfate, with or without added strontium chromate, to maintain the sulfate ion concentration in chromic acid baths containing about 200 grams per liter of CrO or less. Passel improved on this by also using with saturation concentrations of strontium sulfate, sparingly soluble potassium silicofluoride (fluosilicate). Stareck made further im provements by using with the saturation concentrations of strontium sulfate and potassium silic-ofluoride, mixed suppressing agents such as strontium chromate and potassium dichromate to better control the concentrations of sulfate and silicofluoride (fluosilicate) ions by common ion effects. Instead of fluosilicate, Stareck also employed fluoaluminate, fluotitanate and fluozirconate ions in combination with controlled amounts of sulfate ion.

One of the drawbacks of the acidic hexavalent chromium plating baths whenused in decorative plating and containing only sulfate anion as catalyst is that chromium plate will often not cover the bright nickel undercoat in deeply recessed areas when the ratio of chromic acid to sulfate ion is about 100 to 1 or less. When the ratio is greater, for example 150 to 1, the covering power in recessed areas is improved, but where the chromium doesnt cover in deep recesses, an iridescent film is deposited. The use of fluoride ion or complex fluoride ion such as fluosilicate ion in these high ratio chromic acid to sulfate ion baths will help prevent the formation of these unsightly iridescent films or stains where the chromium plate leaves off.

It has now been found that the chromium plating baths containing only sulfate ion as a catalyst or mixtures of In this connection, reference is made to US.

sulfate ion and fluoride ion or complex fluoride ion and providing the ratio of chromic acid anhydride to sulfate is greater than 100 to 1, are helped in covering power improvement and prevention of staining or iridescent film formation in the recess by the presence of fluorocarbon phosphonic acids or mixtures of fluorocarbon phosphonic acids with fluorocarbon sulfonic acids. These fluorocarbon phosphonic acids probably do not act as catalyst in the chromium plating bath, but appear to act as cathodic activators of passive metal surfaces such as those of nickel, iron, copper, brass, stainless steel and chromium.

The standard or conventional chromium bath for plating on nickel, iron, yellow brass or copper and using only sulfate anion as catalyst, is the 100 to 1 ratio of chromic acid :anhydride (CrO to sulfate ion. Thus, in a 200 gram/ liter CrO bath, 2 grams/ liter of sulfate ion would be used, and in a 400 gram/liter CrO bath,-4 gramsv per liter of sulfate ion would be'used. If one uses instead a ratio of 200 to l .of Gro /S0 and plates on top of nickel, copper, yellow brass or steel, only an iridescent chromium chromate (rainbow) film instead of chromium plate is obtained. However, if one plates on a white brass plate consisting of the alloy composition of percent zinc and 20 percent copper with the 200 to 1 ratio chromic acid bath, a bright chromium plate is obtained. It is believed that this White brass plate has a very low passivity, whereas the surfaces of nickel, copper, yellow brass and steel have greater passivity and the baths with the ratio of 200 to 1 chromic acid to sulfate do not have suflicient acidic sulfate ion for activating these passive surfaces. In particular, if chromium plating is attempted on top of a freshly plated bright nickel surface, from an aqueous solution containing, for example, 340 grams/liter of chromic acid and saturated with strontium sulfate at, for example about 5052 C., and using dead entry into this chromic acid bath (that is, the direct plating current is turned on only after the rinsed nickel plated panel is immersed in the chromic acid bath), no chromium plate is obtained on the nickel surface, only a non-metallic iridescent film of basic chromic chromate results.

It has now been found that the addition of from about 0.5 to 10 grams per liter of a fluorocarbon phosphonic acid ,to hexavalent chromium plating solutions allows easily controlled plating baths with excellent covering power. For example, including a fluorocarbon phosphonic acid in the chromium acid plating bath containing saturation concentrations of strontium sulfate as described above, a bright chromium plate is now obtained on the bright nickel panel with the same dead entry as before, instead of the non-metallic iridescent film. This can be best appreciated by chromium plating a bright nickel plated panel in Hull cell tests, using about 5 amps. on the 10 cm. (4 in.) length panel in about 270 cc. of the above baths. Using a mixture of fluorocarbon phosphonic acid with fluorocarbon sulfonic acid is even more effective.

A wide variety of fluorocarbon phosphonic acids may be used in the chromium plating baths of this invention including those containing any of the elements selected from the group consisting of fluorine, phosphorous, carbon, hydrogen and oxygen. A preferred group of acids is alkyl fluorocarbon phosphonic acids of the following formula:

wherein n is an integer of from 1 to 5 inclusive, preferably l or 2. Thus outstanding embodiments of the invention are acids containing 2 or 4 carbon atoms, each carbon atom bearing two fluorine atoms, and the terminal carbon atom in addition bearing a hydrogen atom. One method of preparing these phosphonic acids is to react tetr-afluoroethylene with a dialkyl phosphite in the presence of a free radical-producing catalyst. The reaction product comprises alkyl fluoroalkane phosphonates which then is hydrolyzed to the corresponding fluoroalkane phosphonic acid by treatment with a strong acid, see for example U.S. 2,559,754.

Outstanding results are obtained with chromium plating solutions having a CrO' /SO ratio of from about 10011 to 300:1 and containing from about 0.5 to 10 grams/ liter of a fluorocarbon phosphonic acid.

Preferably the baths contain saturation concentrations of strontium sulfate and thus have a controlled amount of S anion at all times. The baths may also contain fluorocarbon sulfonic acids as described in U.S. 2,750,334.

The fluorocarbon phosphonic acids or mixture with fluorocarbon sulfonic acids added to the pure chromic acid solution without the strontium sulfate present produce no chromium electroplate. That is, with dead entry, neither the saturated strontium sulfate alone nor fluorocarbon acid alone make possible the electrodeposition of chromium from the chromic acid bath. However by the combined use of strontium sulfate and a fluorocarbon phosphonic acid, a bright chromium electroplate of excellent covering power can be obtained. As an example of the latter case in the Hull cell test that was mentioned, the bright nickel plated panel after dead entry is covered with bright chromium electroplate from the highest current density edge down to near the lowest with about 80% of the Hull cell plate covered. This is extremely good chromium coverage on nickel after dead entry. In contrast to the foregoing, it was found that phosphoric acid and phenyl phosphonic acid were ineffective, and, unlike the fluorocarbon phosphonic acid, produced no synergistic effects with saturated strontium sulfate on dead entry of the rinsed nickel plate into the chromic acid solution. That is, in these cases no chromium electroplate resulted after dead entry.

It appears that the role or function of the fluorocarbon phosphonic acid in its cooperative effect with the sulfate anion in the chromic acid baths is that of a cathodic activator of passive metal surfaces such as those of nickel, copper, yellow brass and steel, during the start of the chromium electrodeposition. These metals which are easily passivated by air or by the oxidizing agent chromic acid, seem to be cathodically activated by the fluorocarbon phosphonic acids of this invention. However, there also might be helpful complexing affects with the trivalent chromium formed at the cathode. Thus it appears that slightly soluble strontium sulfate is normally too low to activate the nickel or copper or brass surface on dead entry. However, in the presence of the fluorocarbon phosphonic acid which activates metals like nickel, copper, ibrass, chromium or stainless steel, the S0 anion carries out its function as a catalyst. Since high concentrations of the fluorocarbon phosphonic acids are not critical to the chromium plating, unlike excessive concentrations of sulfate ion which will prevent the electrodeposition of chromium, it is now possible with the conjunctive use of the fluorocarbon phosphonic acids and low sulfate ion concentration to obtain the maximum activation of the metal surface to be chromium plated. This combination makes possible the achievement of excellent chromium plate coverage as well as optimum properties of the chromium plate with respect to porosity, tendency to stresscracking, etching of steel during thick chromium plating (hard chrome plating) of selected areas of steel while purposely shading other areas to prevent deposition of chromium in these areas.

In a bath containing 340 grams/liter chromic acid anhydride, saturated with excess strontium sulfate at 125 F., the ratio of chromic acid to sulfate ion was found to be about 150 to 1, whereas with concentrations of 400 grams/ liter of chromic acid, the ratio was changed up to about 200 to 1. With ratios of chromic acid to sulfate of 200-350 to 1, it is preferred to have a fluoride or fluosilicate ion concentration of about 0.2 to about 1 gram/ liter in addition to a fluorocarbon phosphonic in a concentration of at least 1 gram/liter. It was found that a minim-um concentration of about 0.5 gram/liter of the fluorocarbon phosphonic acids must be present but concentrations of from about 1 to 10 grams/literare preferred.

The foregoing baths can also contain a fluorocarbon sulfonic acid such as perfluoromethyl sulfonic or the sur face-active types such as perfluoro para ethyl (or methyl) cyclohexyl sulfonic acid. These fluorocarbon sulfonic acids can be used in concentrations of from about 0.1 to as high as 10 grams/liter.

The acidic hexavalent chromium plating baths may be made up not only from chromic acid anhydride or chromic acid but also mixtures with dichromates, chromates and polychromates can be used. It is generally preferred to use straight chromic acid or chromic acid anhydride. The presence of cations such as Na, K, Li, Mg, Ca are not detrimental, but they are best kept in low concentration values, that is, it is best that they enter the bath as salts of the fluorocarbon acids, and not by adding high concentrations of, for example, sodium, potassium or calcium dichromate. High concentrations of Na or K ions tend to salt-out certain of the least soluble surface-active fluorocarbon acids.

The chromium plating baths of this invention are further enhanced by the presence of boric acid. Saturation concentrations of boric acid are best, which are about 30 to 50 grams/liter, though concentrations of boric acid as low as 1 to 5 grams/liter show a small beneficial effect. The boric acid is best used when no fluoride or complex fluorides are present in the bath or when excessive concentrations of fluoride are present (over about 1 gram/ liter of fluoride ion).

The following chromium plating bath compositions are representative examples of the baths of this invention.

Example I 200-400 grams/liter of CrO Saturation concentrations of SrSO 0.5 to 5 grams/liter of HCF CF -PO(OH) 5 grams/liter perfluoro para ethyl cyclohexyl sulfonic acid Boric acid 10-50 grams/liter Temperature -l40 F.

Example 11 200-400 grams/liter of CrO Chromic acid anhydride to sulfate ratio of :1 to 300:1

Concentrations of fluosilicate ion of 0.2 to 1 gram/liter 3 grams/liter H(CF CF PO(OH) (Na, K, Li, Mg or Ca salt) 3-5 grams/ liter of perfluoro para ethyl cyclohexy sulfonic acid (Na, K, Li, Mg or Ca salt) Temperature 100-150 F.

This bath has excellent covering power on dead entry Example III -400 grams/liter of CrO Saturation concentrations of SrSO or ratio of CrO, to

S0,, of about 140:1 to about 250:1

Concentrations of fluosilicate ion of 0.2 to about 1 gram/ liter 2-3 grams/liter H(CF CF PO(OH) 5 grams/liter CF SO H.

1-2 grams/liter perfluoro para ethyl cyclohexyl sulfonic acid Temperature 100-150 F.

Example IV 150-400 grams/liter of CrO Saturation concentrations of SrSO, Saturation concentrations of H BO 10 grams/liter of H(CF CF PO(OH) Te p rature 100-150 F.

The bright chromium plate obtained from baths of this invention when applied over bright nickel gives much better corrosion protection in salt-spray tests, than plate from the conventional chromium bath of 100 to 1 ratio of chr-omic acid to sulfate. Also the results in marine ex posure are better. This is probably due to the fact that the surface of the chromium plate has a superior invisible chromate film on it than the chromium plate from the 100 to 1 ratio bath.

When the soluble fluorocarbon sulfonic acids that contain the perfluorocyclohexyl ring, especially with one or two. methyl groups or an ethyl group, are used in concentrations of about 1 to grams/liter, then the chromic acid mist and spray evolved during plating is reduced to zero. The fluorocarbon sulfonic acids containing less than four carbon atoms have little surface activity, that is, very little lowering of the surface tension of the chromic acid bath occurs with 10 grams/liter concentrations. Surface-active agents that are non-fluorinated may be used with these short chain fluorocarbon acids to prevent mist and spray from these baths, but they are not stable to the powerful anodic oxidation at the lead anodes.

The best concentration of chromic acid to use to obtain maximum covering power with the conjunctive use of saturated strontium sulfate is about 340 grams/liter (45 oz./ga1.). However, concentrations as low as about 75 grams/liter and as high as at least 500 grams/liter can be used. The best operating temperature for bright chromium plating is usually from about 100 F. to about 140 F., though in general temperatures of about 50 F. or lower to at least 160 F. and higher can be used, though at the extremes in temperature, the plates are less bright.

As mentioned previously ratios of chromic acid an- I hydride to sulfate of from about 100 to 1 to 300 to 1 are optimum with the use of the fluorocarbon phosphonic acids in concentrations of about 0.5 to 10 grams/ liter. If the chromic acid anhydride to sulfate ratio is much above about 200 to 1, for example 300 to 500 to 1, then fluoride and preferably a complex fluoride anion such as fluosilicate should be present in a concentration of about 0.2 to 1 gram/liter.

What is claimed is:

1. An aqueous acidic hexavalent chromium bath for the electrodeposition of bright chromium plate which contains dissolved therein about 0.5 gram/liter to 10 grams/ liter of a bath soluble fluorocarbon phosphonic acid.

2. A bath in accordance with claim 1 wherein said acidic hexavalent chromium solution has a hexavalent chromium concentration equal to about to about 500 5 3. A bath in accordance with claim 2 wherein said fluorocarbon phosphonic acid is present in a concentration of from 110 grams per liter and has the formula H(CF CF PO(OH) wherein n is an integer equal to 1-5 inclusive.

4. A bath in accordance with claim 2 wherein said bath also contains a fluorocarbon sulfonic acid that has a perfluoro cyclohexyl ring in a concentration of 0.1 to 10 grams/liter.

5. A bath in accordance with claim 1 containing therein an amount of strontium sulfate in excess of its solubility.

6. A method for electrodepositing bright chromium plate which comprises the step of electrolyzing an aqueous acidic hexavalent chromium solution which contains dissolved therein about 0.5 gram/liter to 10 grams/liter of a bath soluble fluorocarbon phosphonic acid.

7. A method in accordance with claim 6 wherein said acidic hexavalent chromium solution has a hexavalent chromium concentration equal to about 100 to about 500 grams/liter CrO and a CrO /SO ratio of from about 100-300 to 1.

8. A method in accordance with claim 6 wherein said fluorocarbon phosphonic acid has the formula wherein n is an integer equal to 1-5 inclusive.

9. A method in accordance with claim 7 wherein said bath contains a fluorocarbon sulfonic acid in a concentration of 0.1 to 10 grams/liter.

10. A method in accordance with claim 7 wherein said bath contains therein an amount of strontium sulfate in excess of its solubility.

References Cited by the Examiner UNITED STATES PATENTS Johnson 20451 JOHN H. MACK, Primary Examiner. KAPLAN, Assistqnt Examiner, 

1. AN AQUEOUS ACIDIC HEXAVALENT CHROMIUM BATH FOR THE ELECTRODEPOSITION OF BRIGHT CHROMIUM PLATE WHICH CONTAINS DISSOLVED THEREIN ABOUT 0.5 GRAM/LITER TO 10 GRAMS/ LITER OF A BATH SOLUBLE FLUOROCARBON PHOSPHONIC ACID. 